The present invention relates to a power conversion device for a DC power and an AC power and a power-conversion control unit of the same.
Size and weight reduction of an isolation transformer for system interconnection has been difficult, because the transformer is driven at a low frequency of several tens of Hz that is the same as a system frequency. Applying an SST (Solid State Transformer) to a large, high voltage power use has been examined recently. The SST is configured by a combination of a high-frequency transformer and a power circuit, such as a DC/DC converter and an inverter, that drives the high-frequency transformer and inputs or outputs an AC current having the same frequency as that of the conventional one. The SST substitutes for the conventional transformer. The high-frequency transformer is driven at a high frequency of several tens to several hundreds of kHz, thereby the size and weight can be largely reduced even with the configuration of the SST in which the power circuit is added, as compared with the conventional transformer alone.
Introduction of natural energy, e.g., photovoltaics and wind-power generation, has been expanding globally and, as a use of a power converter for a system, a high-performance PCS (Power Conditioning System) has been demanded that can control an electric power of the nature energy and output the power to the system. A high-voltage isolation transformer is used for the output, which has to be driven at a low frequency of several tens of Hz that is the same as the system frequency. Therefore, equipment becomes larger.
Some power converters using a high-voltage power, which are for a high-voltage motor or pump and railway, for example, also use a high-voltage isolation transformer for an input. In the case of using the high-voltage isolation transformer for the input, as in the case of using it for the output, the high-voltage isolation transformer is driven at a low frequency of several tens of Hz that is the same as the system frequency by receiving an electric power from the system. Therefore, the equipment becomes larger.